Interactions of Coexisting Neurotransmitters and Effects of Chronic Drug Treatment

Abstract

The phenomenon of coexistence of classical neurotransmitters and neuroactive peptides has been demonstrated in a number of cases in the central and peripheral nervous system (cf. Hökfelt et al., 1980; Cuello, 1982; Lundberg and Hökfelt, 1983). In the descending bulbospinal pathway originating from the raphe nuclei, coexistence of at least three putative neurotransmitters; the undecapeptide substance P (SP), the tripeptide thyrotropin-releasing hormone (TRH) and serotonin (5-hydroxytryptamine, 5-HT) has been shown using immunohistochemical methods (Chan-Palay et al., 1978; Hökfelt et al., 1978; Johansson et al., 1981). Treatment with the serotonergic “neurotoxins” 5,6- or 5,7-di-hydroxytryptamine results in an almost complete loss of 5-HT, TRH and SP immunoreactive fibers in the ventral spinal cord (Björklund et al., 1979; Johansson et al., 1981; Gilbert et al., 1982; Marsden et al., 1982) although findings suggesting a limited scope of coexistence only, were also reported (Towle et al., 1986).